1. Field of the Invention
This invention relates to an improved MOS structure having reduced hot carrier characteristics. More particularly, this invention relates to an improved MOS structure having a trench in the substrate adjacent the gate electrode to permit deeper source/drain doping of the substrate adjacent the gate electrode to inhibit electron penetration of the gate oxide, and a process for forming such a structure.
2. Description of the Related Art
In conventional CMOS structures, the N channel transistors are known to experience gate oxide damage during operation due to hot carriers. Hot carriers are electrons traveling through the channel beneath the gate oxide from the source to the drain which, when encountering high fields adjacent the drain region, tend to migrate toward and into the gate oxide region above the channel, instead of toward the drain region, as a result of the high energy imparted to the electron by the high fields. Such electrons, known as hot carriers, damage the gate oxide resulting in a degradation in performance of the transistor and the MOS structure.
Efforts in the past to remedy this problem have included the placement of lightly doped drain (LDD) regions between the drain region and the channel, i.e., in the portion of the substrate beneath the oxide spacer on the sidewall of the gate electrode. Attempts to solve the problem have also included driving the source/drain lateral junction with the channel region deeper, i.e., diffusing the source/drain dopants deeper into the substrate. However, the disadvantages of these old methods are lack of localized control of the dopant distribution of LDD and loss of source/drain profile around the gate.
It would, therefore, be desirable to provide an MOS structure wherein electrons traveling through the channel beneath the gate oxide from the source to the drain of an MOS device do not enter and damage the gate oxide of the MOS device when encountering high electric fields adjacent the drain region, while avoiding the problems encountered by the prior art when attempting to solve the hot carrier problem.